1. Field of the Invention
The present invention relates to a twin-roll continuous caster by which a cast strip can be directly produced from molten metal. More specifically, it relates to a control device and a control method for the twin-roll continuous caster, which device and method enable the production of a cast strip with high-quality surfaces.
2. Description of the Related Art
In the well-known twin-roll casting process, molten metal is continuously supplied into a molten pool defined between a pair of opposed cooling rolls which rotate in opposite directions, and on each cooling roll a solidified shell is formed by contact between the molten metal and the cooling roll, and thus the solidified shells are bonded at the nearest point of contact of each of the rolls, i.e., a kissing point, to thereby produce a cast strip.
Furthermore, Japanese Unexamined Patent Publication No. 60-64754 discloses a method of eliminating bulging, which occurs during bonding when the roll separating force is low, and to prevent roll slip, which occurs during bonding when the roll separating force is high. Note that bulging results in an unbonded condition of the shell, thereby causing a separation or break out of the cast strip.
In the above method, first a rolling load of the solidified shells, as a force reacting against the roll separating force, is detected, and then a solidification period of the shells between the cooling rolls, which can be representative of either a rotating speed of the cooling rolls or a height of the molten pool, is controlled in such a manner that the rolling load is neither too high nor too low.
Note that, in addition to the above method, Japanese Unexamined Patent Publication Nos. 59-56950, 60-92051, 61-232044, 61-232045, 61-289950, 62-97749 disclose methods or devices for eliminating bulging.
In general, when solidified shells having a given thickness are bonded at the kissing point, the greater the increase of the roll separating force the stronger the binding strength, but when the roll separating force is higher than a predetermined value, many continuous surface cracks extending in the casting direction are produced in the cast strip.
This surface crack phenomenon is due to a local stress concentration generated of the solidified shells when rolling solidified shells having an unequal thickness in the longitudinal direction of the cooling roll. Note, the thicker the target thickness of the cast strip or the higher the roll separating force, the greater the incidence of continuous surface cracks due to larger variations of thickness of the solidified shell. Further, it has been found that surface cracks still occur even when the roll separating force is lower than the roll separating force value at which the afore-mentioned roll slip phenomenon occurs. Therefore, the method of controlling the solidifcation period as disclosed in Japanese Unexamined Patent Publication No. 60-64754, can not prevent the occurence of continuous surface cracks. Further, although the object of Japanese Unexamined Patent Publication No. 62-97749 is to prevent the occurrence of surface cracks by detecting and controlling the roll separating force, it does not consider the influence of the cast thickness upon the occurrence of surface cracks.